Systems for treating water using iron

ABSTRACT

Systems and associated methods for treating contaminant-containing wastewater are provided. The systems generally include a reducing zone for reducing the oxidation-reduction potential of the water and a clean-up zone comprising zero valent iron for removing at least a portion of the contaminant from the contaminant-containing water. The systems are operable to remove one or more contaminants from the contaminant-containing water and are operable for extended durations without clogging due to the formation of iron hydroxides.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of and claims priority to U.S.patent application Ser. No. 11/749,448 filed May 16, 2007, now U.S. Pat.No. 7,897,049, entitled, “METHODS FOR TREATING WATER USING IRON”; claimspriority to U.S. Provisional Patent Application No. 60/803,626 filed May31, 2006, entitled “SYSTEMS AND METHODS FOR TREATING WATER USING IRON”,and is related to and claims priority to PCT Application No.PCT/US2007/69016 filed May 16, 2007, entitled “SYSTEMS AND METHODS FORTREATING WATER USING IRON”, each of which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to systems and methods for treating water,such as groundwater, wastewater and the like, using iron.

BACKGROUND OF THE INVENTION

Water generated in industrial processes and groundwater in thesubsurface environment may contain various kinds of contamination,including, for example, cyanide, arsenic, chromium, selenium andhalogenated organics, to name a few. Various in-situ systems fortreating such contaminant-containing water using iron are known in theart. For example, U.S. Pat. No. 5,266,213 to Gillham, U.S. Pat. No.5,362,394 to Blowes et al., and U.S. Pat. No. 5,534,154 to Gillham alldisclose the use of various trench systems that include elemental ironto remove impurities from water flowing therethrough. These patents allrequire extremely low oxygen levels and long residences times tofacilitate the removal of contaminants within the water. It is notalways possible to treat contaminated water in an oxygen freeenvironment. Moreover, long residence times restrict clean waterproduction rates.

Various ex-situ treatment systems utilizing iron to remove contaminantsfrom water are also known. For example, U.S. Pat. No. 5,266,213 toGillham and U.S. Pat. No. 5,534,154 to Gillham both disclose the use oftank systems comprising elemental iron and/or activated carbon to treatcontaminated water. Again, these patents both require extremely lowoxygen levels within the tanks and long residence times. U.S. Pat. No.5,837,145 to Dzombak et al. discloses the use of a fixed permeable bedof iron and sand to remove cyanide from water. These systems suffer inthat, over time, iron hydroxides will precipitate into a sludge andcause clogging.

Despite these drawbacks, it is still desirable to utilize elemental ironin water treatment. Elemental iron is relatively inexpensive, widelyavailable and is highly reactive with many of the contaminants withinwater. Thus, there exists a need for improved methods and systems fortreating contaminated water using elemental iron.

SUMMARY OF THE INVENTION

In view of the foregoing, a broad objective of the present invention isto enable the prolonged removal of contaminants from wastewater usingelemental iron without the need to restrict dissolved oxygen levels.Another objective is to provide high-throughput systems adapted toprovide ex-situ and/or in-situ removal of contaminants using elementaliron. A further objective is to enable the prolonged use of such systemsand associated methods without forming a significant amount of ironhydroxides.

In addressing one or more of the above objectives, the present inventorshave recognized that iron hydroxides are generally formed due to thecorrosion of zero valent iron (“ZVI”/Fe⁰) to Fe²⁺, followed by oxidationof Fe²⁺ to Fe³⁺. The inventors have also recognized that iron hydroxideproduction is restricted in reducing environments. The present inventorshave thus determined that flowing a contaminant-containing water througha reducing zone to reduce the oxidation-reduction (“redox”) potential ofthe contaminant-containing water followed by flowing thecontaminant-containing water through a clean-up zone comprisingzero-valent iron would facilitate the removal of many contaminants inthe contaminant-containing water with restricted production of ironhydroxides.

In one aspect of the invention, an inventive water treatment system isprovided, the system including a reducing zone (e.g., a first zone) anda clean-up zone (e.g., a second zone). The reducing zone generallyincludes an iron-based reducing agent adapted to reduce the redoxpotential of water flowing therethrough. In this regard, the presentinventors have recognized that the corrosion reaction of ZVI to Fe²⁺significantly reduces the redox potential of the water and that thefurther corrosion of Fe to Fe³⁺ is related to the residence time of thewater within the zone. Thus, the residence time of thecontaminant-containing water within the reducing zone is generally ofsufficient duration to enable the iron-based reducing agent to becorroded to Fe²⁺, but the residence time is not so long that significantamounts of Fe³⁺ are produced from the oxidation of Fe²⁺. In other words,the average residence time of the water within the reducing zone may beof sufficient duration to lower the redox potential of thecontaminant-containing water, thereby significantly reducing the rate ofFe²⁺ oxidation and hence the quantity of corrosion products (e.g.,amorphous iron oxides and hydroxides). Preferably, the residence time inthe reducing zone is sufficient to lower the redox potential of thewater exiting the reducing zone to not greater than 0 millivolts, suchas not greater than −50 millivolts, preferably not greater than −100millivolts, or even not greater than −200 millivolts. As noted, suchreduction conditions will restrict Fe³⁺ production, and correspondinglyiron hydroxide production.

The residence time of the contaminant-containing water within thereducing zone is related to a variety of factors, such as the desiredredox potential, described above, the total mass of iron-based reducingagent within the reducing zone, and the total surface area of theiron-based reducing agent within the reducing zone, to name a few. Suchresidence times and other corresponding variables are within the scopeof the present invention insofar as the redox potential of thecontaminant-containing water is lowered and with insubstantial Fe³⁺production.

In one embodiment, the iron-based reducing agent comprises ZVI particlesand the reducing column includes not greater than 10 wt % ZVI, such asnot greater than 5 wt % ZVI, but at least about 1 wt % ZVI. In thisembodiment, the ZVI particles have an average diameter of at least about0.1 mm to not greater than about 1.2 millimeters, and often an averagediameter of at least about 0.35 mm to not greater than about 0.65 mm.Accordingly, the ZVI particles have an average surface area of at leastabout 0.03 mm² to not greater than about 3.8 mm², and often an averagesurface area of at least about 0.4 mm² to not greater than about 1.3mm². Thus, in this embodiment, the total available ZVI surface area isgenerally at least about 450 mm² to not greater than about 55,000 mm².In this embodiment, the present inventors have found that the averageresidence time of the contaminant-containing water within the reducingzone should be at least about 45 minutes, such as at least about 60minutes, or even at least about 75 minutes. The residence time, however,should be restricted to prevent the formation of Fe³⁺ and ironhydroxides. Hence, in this embodiment, the residence time is generallynot greater than 4 hours, such as not greater than 3 hours, or even notgreater than 2 hours. Thus, the system is capable of relatively highcleaning rates relative to prior art systems, which require extendedresidence times to achieve the desired contaminant removal rates.

As noted, the residence time may be related to the mass of ZVI in thereducing zone. For reducing zones comprising at least about 1 wt % tonot greater than about 10 wt % ZVI, the residence time per mass of ZVIshould be at least about 1 minute per kilogram ZVI, such as at leastabout 2 minutes per kilogram ZVI, or at least about 3 minutes perkilogram ZVI, or even at least about 4 minutes per kilogram ZVI. Theresidence time per mass of ZVI should not exceed 25 minutes perkilogram, such as not greater than 20 minutes per kilogram, or even notgreater than 15 minutes per kilogram, or even not greater than 10minutes per kilogram.

The iron-based reducing agent may include materials other than ZVIparticles. For example, the reducing agent may also/alternativelycomprise other iron containing media such as bauxite residue, ironshavings/borings and/or bauxite. Correspondingly, the residence time ofthe contaminant-containing water may vary according to the utilizedreducing agent(s) and reaction system utilized (e.g., plug-flow orCSTR).

The reducing zone may include other materials. For example, the reducingzone may include a filler material for facilitating flow of thecontaminant-containing water through the reducing zone while enablingsufficient contact between the reducing agent and contaminant-containingwater. The filler material may include, for instance, sand.

Any suitable amount of reducing agent and/or filler may be used withinthe reducing zone so long as the goal(s) of lowered redox potentialand/or restricted Fe³⁺ production is/are accomplished. For instance, thereducing zone may include not greater than 1 gram of reducing agent(e.g., ZVI) per 10 grams of filler (e.g., sand), such as not greaterthan 0.5 gram of reducing agent per 10 grams of filler, or even notgreater than 0.2 gram of reducing agent per 10 grams of filler. In oneembodiment, the reducing zone includes not greater than 10 wt % ZVI,such as not greater than 7.5 wt % ZVI, or even not greater than 5 wt %ZVI. In this embodiment, the reducing zone should include at least about0.5 wt % ZVI, such as at least about 1 wt % ZVI.

As noted, the reducing zone acts to lower the redox potential of theeffluent water of the reducing zone. The reducing zone may also serveother functions. For example, the reducing zone may also act as apre-cleaning zone, wherein Fe²⁺ corroded from elemental iron may reactwith contaminants within the contaminant-containing water. Hence, insome instances, the reducing agent may serve as both a reducing agentand a cleaning/binding agent, and the reducing zone may serve as both areducing zone and a contaminant removal zone.

As noted above, the inventive system also includes a clean-up zone. Theclean-up zone is downstream of the reducing zone and includes ZVI tofacilitate removal of contaminants from the contaminant-containing water(e.g., via precipitation). Since the redox potential of the incomingcontaminant-containing water has been sufficiently lowered via thereducing zone, the clean-up zone may include a large amount of ZVI tofacilitate water treatment. For example, the clean-up zone may includeup to 100 wt % ZVI within the zone, such as at least about 50 wt % ZVI,or even at least about 75 wt % ZVI. The clean-up zone may also includeother materials, such as a filler material (e.g., sand) to occupy theremaining volume of the clean-up zone.

The residence time within the clean-up zone is generally similar to theresidence time within the reducing zone, and thus may include any of theresidence times discussed above. However, the residence time within theclean-up zone may be shortened or extended, as necessary, to achieve thedesired cleaning rates (e.g., shortened or extended relative to anaverage amount of contaminants contained within thecontaminant-containing water).

The reducing zone and clean-up zone of the treatment system may beconfigured in a variety of manners. For example, an ex-situ system mayinclude a first column comprising the reducing agent and a second columncomprising the ZVI. Alternatively, an ex-situ system may include asingle column comprising both zones separated by a suitable boundary(e.g., sand, a membrane). Other ex-situ systems (e.g., tanks) may beemployed in accordance with the present invention. In-situ systems, suchas trenches and/or permeable reactive barriers (PRBs), may also beemployed. In one embodiment, the reducing zone and clean-up zone aredistinct zones within separate regions of the treatment system. Inanother embodiment, the reducing zone and clean-up zone are partiallyoverlapping.

The zero valent iron within the clean-up zone and, optionally, thereducing zone, may be of any suitable physical form, such as ironfilings, iron pellets, shredded scrap iron, iron cuttings, and irondust. In one embodiment, the ZVI is of a granular form and has anaverage particle diameter of at least about 0.1 mm to not greater thanabout 1.2 mm, as described above.

The present system and methods are advantageous in that low dissolvedoxygen levels within the water are not required for operation. Thepresent inventors have found that, while low dissolved oxygen levels arepreferred, the present system is capable of cleaningcontaminant-containing water that contains normal dissolved oxygenlevels (e.g., 3-7 mgIL dissolved oxygen) for extended periods of time.

The pH of the incoming water should generally be not greater than 8,such as a pH of at least about 5 to a pH of not greater than 8, such asa pH in the range of from about 6 to about 8. If necessary, the watermay be pretreated with an acid to reduce the pH of the water enteringthe reducing zone.

These and other aspects, advantages, and novel features of the inventionare set forth in part in the description that follows and will becomeapparent to those skilled in the art upon examination of the followingdescription and figures, or may be learned by practicing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one embodiment of a treatment systemuseful in accordance with the present invention.

FIG. 2 is a flow chart illustrating one embodiment of methods useful inaccordance with the present invention.

FIG. 3 is a graph illustrating cyanide removal results obtained fromoperation of a prior art system.

FIG. 4 is a graph illustrating cyanide removal results obtained from atreatment system produced in accordance with the present invention.

FIG. 5 is a graph illustrating influent and effluent pH and dissolvedoxygen levels obtained from a treatment system produced in accordancewith the present invention.

FIG. 6 is a graph illustrating influent and effluent cyanide levelsobtained from a treatment system produced in accordance with the presentinvention.

FIG. 7 is a graph illustrating influent and effluent selenium levelsobtained from a treatment system produced in accordance with the presentinvention.

DETAILED DESCRIPTION

Reference will now be made to the accompanying figures, which at leastassist in illustrating various pertinent features of the presentinvention. One embodiment of a treatment system is illustrated inFIG. 1. In the illustrated embodiment, the treatment system 1 includes areducing column 10 and a clean-up column 20. A contaminant-containingwater source 5 is fluidly interconnected to the inlet of the reducingcolumn 10 via piping 7. The clean-up column 20 is fluidly interconnectedto the outlet of the reducing column 10 via piping 12. An optional thirdtreatment zone 30 may be fluidly interconnected to the outlet of theclean-up zone 20 via piping 14, as discussed in further detail below.One or more pumps 3 may be utilized to facilitate flow of thecontaminant-containing water through the system 1, as indicated by flowdirection arrow 4. Pressure gauges 16 may be utilized to monitor thepressure within one or more of the columns 10, 20. While the system hasbeen illustrated with a single reducing column and a single clean-upcolumn, any number of column(s), or similar apparatus, could be used.

In operation, contaminant-containing water 6 from a source 5 flows intoand through the reducing column 10, where the redox potential of thewater 6 is reduced via interaction with an iron-based reducing agent(e.g., ZVI) of the reducing column 10. Preferably, the residence time ofthe water within the reducing column 10 is sufficient to lower the redoxpotential of the water exiting the reducing column 10 to not greaterthan 0 millivolts, but is not so long as to promote the formation ofiron hydroxides.

After reduction oxidation potential of the water in the reducing column,the water 6 flows to the clean-up column 20, which contains at least 50wt % ZVI, where contaminants within the water react with the Fe²⁺ and/orFe³⁺ and are precipitated out of the water 6. More particularly, Fe²⁺may react with one or more of cyanide, chromium, arsenic, selenium,radionuclides, pathogens and/or halogenated organics (e.g., chlorinatedorganics such as chloroethenes, chlorobenzenes, and chloromethanes)within the water 6. For example, Fe²⁺ may react with cyanide complexesto form one or more of Prussian Blue, Turnbull's Blue and/or BerlinWhite. Since ZVI also reacts well with many bacteria and viruses, thetreatment system 1 can also be employed to disinfect water that containssuch bacteria and/or viruses. Thus, the system 1 is adapted to removal aplurality of different contaminants from contaminant-containing water.

The water 6 may also pass through one or more filters 60 to filter outany large contaminants within the water. After treatment in the clean-upcolumn 20 and, optionally, the third treatment zone 30, the treatedwater is passed to a reservoir 70 (e.g., via piping 14) or is sent toanother facility.

As noted, the system 1 may include an optional third treatment zone 30for further removing contaminants from the water 6. For example, thethird treatment zone 30 may include a scavenging material, such as abiological material (e.g., compost materials, such as spent mushrooms orleaves) and/or activated carbon, among others. The third treatment zone30 is generally fluidly interconnected to the outlet of the clean-upzone 20.

As noted above, the reducing column may also facilitate removal ofcontaminants. For example, the reducing conditions produced in thereducing column may reduce chromium (VI) to chromium (III), which willbe precipitated out as chromium oxide and/or chromium hydroxide.

As noted above, the water treatment system 1 may be utilized fortreating a variety of contaminant-containing waters. In a particularembodiment, the treatment system 1 may be utilized to treat industrialwastewater from aluminum smelting activities to remove free cyanide andassociated metal cyanide complexes. It has been found that systemsemploying the above-described dual zone approach are capable of removingnearly all total cyanide within contaminant-containing water, includingnearly all free cyanide, over substantial periods of time and porevolumes. Indeed, such systems may be capable of removing at least about90 wt % of the total cyanide within the contaminant-containing waterover extended periods of continuous operation, such as removal of atleast about 95 wt % total cyanide, or even at least about 97 wt % totalcyanide, and, in some instances, at least about 99 wt % of the totalcyanide from the contaminant-containing water. The present system isalso capable of removing at least about 95 wt % free cyanide fromcontaminant-containing water over extended periods of continuousoperation, such as removal of at least about 97 wt % free cyanide, oreven at least about 99 wt % free cyanide, and, in some instances, atleast about 99.5 wt % free cyanide from the contaminant-containingwater. The effluent exiting the treatment system 1 thus generallycontains very low levels of cyanide, such as not greater than 100 ppbtotal cyanide, or such as not greater than 50 ppb total cyanide, or suchas not greater than 25 ppb total cyanide, or such as not greater than 12ppb total cyanide. These removal rates may be achieved over extendperiods of operation without significant production of iron hydroxides,such as over at least about 4 weeks, or even at least 11 weeks and/or atleast 1050 pore volumes, or even at least about 7500 pore volumes.

In another aspect of the invention, an inventive method of treatingcontaminant-containing water is provided. With reference to FIG. 2, themethod generally includes the steps of passing contaminant-containingwater through a reducing zone, lowering the redox potential of thewater, flowing the water through a clean-up zone comprising zero valentiron, and removing contaminants from the contaminant containing water.The reducing zone includes a reducing agent (e.g., ZVI) and thus thelowering step may include the step of corroding the reducing agent(e.g., Fe⁰→Fe²⁺+2e−) to lower the redox potential of the water andwithout production of substantial amounts of Fe³⁺. In other words, theeffluent exiting the reducing zone may include an insubstantial amountof Fe³⁺ The method may further include the steps of removingcontaminants during the passing and flowing steps, thereby achievingcontaminant removal in both zones. For example, the removing step mayinclude the step of corroding ZVI to Fe²⁺, forming precipitates from theFe²⁺ and contaminants within the contaminant-containing water andremoving the precipitates from the contaminant-containing water (e.g.,via adhesion/interaction with the reducing agent, filler and/or ZVImedia). The method may include any of the configurations and/orarrangements described above for the water treatment system.

EXAMPLES Example 1 Prior Art System

A single column comprising ZVI in sand is constructed in accordance withU.S. Pat. No. 5,837,145. Ferro- and free cyanide spiked synthetic waterresembling aluminum smelting activities impacted groundwater is passedthrough the single-column system and the effluent is routinely monitoredfor total cyanide, pH, redox potential and dissolved oxygen. FIG. 3provides a graphical representation of column performance using thesingle column. The cyanide broke through the column within about 800pore volumes and hydraulic failure was noticeable around 2000 porevolumes.

Example 2 New, Lab-Scale System

A lab-scale system including a reducing column and a clean-up column isfabricated. The reducing column includes 5 wt % ZVI in sand and theclean-up column includes 100% ZVI. Ferro- and free cyanide spikedsynthetic water resembling aluminum smelting activities impactedgroundwater is passed through the reducing column and then the clean-upcolumn and the effluent is routinely monitored for total cyanide, pH,redox potential and dissolved oxygen. The effluent total cyanideconcentration is reduced to non-detectable levels within 1945 porevolumes of operation and remains non-detect over extended periods ofoperation (>7500 pore volumes). FIG. 4 provides a graphicalrepresentation of the achieved cyanide concentration levels relative topore volumes. Insubstantial iron hydroxide production is witnessedduring the operating period. No hydraulic failure occurs during theoperation period.

Example 3 New, Pilot-Scale System

A pilot unit including a reducing column and a clean-up column isfabricated. The reducing column comprises a 26 inch bed of 5 wt % ZVI(CC-1190 ZVI, supplied by Connelly GPM Inc.) in sand (Filpro No. 1 sand)and the clean-up column comprises a 26 inch bed of 100 wt % ZVI (CC-1190ZVI, supplied by Connelly GPM Inc.). Both columns have an internaldiameter of about 6 inches. A vertical upflow stream ofcyanide-containing water (e.g., from an aluminum smelting facility) isflowed through the reducing column and then the clean-up column at aflow rate of about 82 milliliters per minute, on average. The effluentis monitored for pH, dissolved oxygen and cyanide concentrations. Thepilot unit is operated for 11 weeks for a total reactive pore volume of˜1100, during which time 99.6% total cyanide and, statistically, 100%free cyanide, weak metal cyanide complexes and iron cyanides areremoved. Insubstantial iron hydroxide production is witnessed during theoperating period. No hydraulic failure occurs during the operationperiod. FIG. 5 illustrates the influent and effluent pH and dissolvedoxygen levels during the operation period. FIG. 6 is a graphillustrating the achieved cyanide removal during the operation period.Even with extremely high influent cyanide concentrations (e.g., >700ppb), the effluent exiting the system contains less than 30 ppb totalcyanide for the duration of the test, and less than 20 ppb forsignificant periods of operation. In this example, the average flow rateof the contaminant-containing water per cross-sectional area of thecolumn is from about 150 gallons/(day−ft²) to about 500gallons/(day−ft²).

FIG. 7 is a graph illustrating achieved selenium removal utilizing alab-scale unit having a reducing column and a clean-up column, similarto that described above in Example 2. The pilot unit consistentlyachieved a selenium removal efficiency of at least 90% during operationat hydraulic loading rates of about 0.5 gallon per minute per squarefoot. The removal efficiency dropped to about 75% during operation athydraulic loading rates of about 1 gallons per minute per square foot.

The foregoing description is considered as illustrative only of theprinciples of the invention. Furthermore, since numerous modificationsand changes will readily occur to those skilled in the art, it is notdesired limit the invention to the exact construction and process shownand described above. Accordingly, resort may be made to all suitablemodifications and equivalents that fall within a scope of the inventionas defined by the claims which follow.

1. An ex-situ system for treating contaminant-containing water, thesystem comprising: a first treatment zone comprising a reducing agentadapted to reduce the oxidation-reduction potential ofcontaminant-containing water flowing therethrough, wherein the firsttreatment zone comprises an iron-containing media present in an amountof less than about 10 wt % of the first treatment zone, wherein thetreatment zone comprises a pH of at least about 5, wherein said firsttreatment zone is capable of reducing the oxidation-reduction potentialof the contaminant-containing water to not greater than about 0 mV, andrestricting Fe³⁺ and iron hydroxide production; and a second treatmentzone downstream of the first treatment zone, the second treatment zonecomprising at least 50 wt % zero valent iron for removing at least aportion of contaminants contained in the contaminant-containing water;wherein said second treatment zone is capable of corroding zero valentiron into Fe²⁺ using the contaminant-containing water, formingprecipitates from the Fe²⁺ and contaminants of thecontaminant-containing water, and removing precipitates from the waterat a hydraulic loading of about 0.5 gpm/ft².
 2. The system of claim 1,wherein the reducing agent comprises zero valent iron.
 3. The system ofclaim 1, wherein the first treatment zone comprises a filler.
 4. Thesystem of claim 3, wherein the first treatment zone is a column andwherein the column consists essentially of the reducing agent and thefiller.
 5. The system of claim 1, wherein the second treatment zoneconsists essentially of zero valent iron.
 6. The system of claim 1,further comprising: a third treatment zone fluidly interconnected to anexit of the second treatment zone, the third treatment zone comprising acontaminant scavenging material.
 7. The system of claim 6, wherein thecontaminant scavenging material comprises at least one of activatedcarbon and a biological material.
 8. The system of claim 1, wherein thecontaminant-containing water exiting the second treatment zone comprisesless than 30 ppb total cyanide.